DESCRIPTION: The primary objective of this proposal is to test the hypothesis that increases in adenosine production during ischemia cause adaptive changes in key myocardial ATP fluxes which subsequently restore energy balance and delay the onset of irreversible myocardial damage. Effort is focused on two aspects of this feedback system: determining the kinetics and mechanism of the protective effect of adenosine during ischemia, and defining the pathway for adrenergic amplification of adenosine production. Four Specific Aims are proposed: (1) test the hypothesis that the previously established myocardial protective effect of adenosine develops more slowly than ATP depletion to critical levels during acute total ischemia; (2) identify the mechanism of the protective action of adenosine during underperfusion; (3) test the hypothesis that the increase in myocardial adenosine production during underperfusion is not caused by ATP breakdown alone, but that there is amplification of adenosine production due to alpha receptor stimulation through release of stored norepinephrine and (4) test the hypothesis that the mechanism of alpha receptor amplification of adenosine production during underperfusion is related to one of the adenosine/ATP fluxes. Specifically, the latter aim will identify whether adenosine production is amplified due to (a) increased extracellular production of adenosine, (b) activation of cytosolic 5'nucleotidase or inhibition of adenosine kinase or (c) altered membrane transport for adenosine.The experimental plan will exploit the demonstrated utility and feasibility of NMR spectroscopy measurements of high energy phosphate compounds. Key tools include selective transport, receptor and enzyme agonists and antagonists, tracer kinetic technologies and a comprehensive mathematical model of the system.Emphasis is placed on distinguishing the target ATP fluxes from the cellular pathways linking the adenosine receptor and the target. The studies of adrenergic amplification of adenosine production will explore the ATP dependent and independent mechanisms as well as develop an analytical model for maximizing the information yielded from measurements using multiple indicator dilution analysis and enzyme blockers to assess cytosolic adenosine concentrations. Results of the study will be critical for guiding attempts to take therapeutic advantage of the protective effects of adenosine during myocardial ischemia. They will elucidate the relation between myocardial energetics and adenosine may play its most critical role as a local humoral agent.